In recent years, the synthesis of copper (Cu) nanoparticles has become of great importance from a scientific point of view as well as from an industrial point of view; due to its huge potential for replacing expensive nano-silver-based inks. Since the 1990s, many attempts have been made to synthesize nano-copper by wet chemistry processes, as well as by gas or solid phase methods. Among these are the sonochemical method, microemulsion techniques, polyol processes, radiation methods, thermal reduction, reducing flame synthesis, metal vapor synthesis, vacuum vapor deposition and chemical reduction in solution[1-3].
Copper NPs can be mainly used in ink formulations for printing conductive patterns. Copper NPs based inks suffer from two main disadvantages: poor resistance to oxidation and high sintering temperatures. Therefore, sintering of copper NPs is typically carried out under special conditions and atmospheres, such as N2, H2, carboxylic acid vapors and others, and heating to high temperatures which prevent the use of low cost plastic substrates which are temperature sensitive. To date, copper NPs based inks have been sintered at temperatures above 150° C.
Recently, several reports were published on the use of copper complexes as conductive inks [4-9].
Lee et al. [4] reported on water based ink-jet inks based on a copper ion complex for use in printing conductive patterns. These inks consisted of ammonia water soluble Cu-formate or Cu-citrate complexes which are synthesized by electrolysis. Copper electrode patterns made of these ammonia complexes were obtained only after sintering the printed patterns at 250° C. for 60 minutes under hydrogen atmosphere.
Kim et al. [5] reported on a copper-complex ink, formed by mixing copper formate and hexylamine while the hexylamine served both as a complexing agent and as a solvent. The obtained layer was sintered under nitrogen flow with formic acid vapors. Choi et al. [6] have further developed a composition for use as a screen-printing paste which can be sintered under similar conditions (nitrogen followed by a reducing atmosphere).
Yabuki et al. [8] reported on a solvent ink composed of complexes of copper formate tetrahydrate and n-octyl amine dissolved in toluene. The copper electrodes were formed by calcining the printed inks thermally on glass substrates at a temperature range of between 110 and 140° C. under nitrogen atmosphere.
Chung et al. [9] reported the formation of various complex-based conductive inks (based on silver, copper and other metals), that can be formulated by forming complexes with ammonium carbamate and ammonium carbonate. These complexes were further formulated with copper or silver flakes and sintered under nitrogen to give low sheet resistance.